1. Field of the Invention
This invention relates in general to head mounted displays which produce images visible to the wearer of the display, and, more particularly, to a helmet mounted display that operates in conjunction with a partially reflective visor to superimpose the display on an exterior scene that is being viewed by the wearer.
2. Description of the Related Art
Helmet mounted display apparatus finds utility in many areas, such as training, machine control or entertainment. In recent years, such devices have found particular utility in military applications where information must be supplied to vehicle operators and weapons controllers. Aircraft pilots find such devices useful to furnish information relating to aircraft operation, radar presentations, maps, weapons aiming and other things that would be useful or desirable in their mission.
The images, which are superimposed on a surface in the wearer's line of sight, can be derived from a variety of sources including cathode ray tubes (CRTs) that can be mounted in a helmet. Prior art devices have utilized fiber optics displays, flat screen liquid crystal devices with image enhancers, and even photographic projectors operating with an appropriate optical relay system, all of which might supply the wearer with an informational image.
Helmet mounted systems for displaying images are well known in the art. U.S. Pat. No. 3,059,519, to Stanton, taught a helmet mounted display in which a CRT display was directed through an optical system that included a plurality of reflecting mirrors which ultimately placed the image in the eyes of the observer, slightly above the normal line of sight. However, an alternative embodiment included a partially reflective mirror in combination with a parabolic reflector. Another alternative embodiment replaced one of the mirrors with a semi-reflective mirror so that the pilot's view would not be completely obstructed by the helmet and mirrors.
U.S. Pat. No. 3,787,109, to Vizenor, disclosed a display where the helmet was provided with a paraboloidal visor with a partially reflective coating on the inner surface as the primary optical element in transmitting the image from the generating device to the eye of the observer. Such an innovation eliminated the need for separate combiner screens or beam splitters near the eyes.
A more complex optical path that still utilized a visor was disclosed in U.S. Pat. No. 3,923,370, to Mostrom. In this patent, limited areas of the inner surface of the visor were coated to be partially or totally reflective. An image generating system mounted at the upper portion of the helmet projects an image to a first reflective area which is high on the visor. The image is relayed to a planar mirror mounted on the front of the helmet and from there to a second reflective area which is just above the normal line of sight of the observer. As a result, all of the reflective elements are deemed to be substantially out of the field of view of the helmet wearer.
A similar optical arrangement is also shown in U.S. Pat. No. 4,465,347, to Task et al. However, the image which is observed in the disclosed system is derived from a telescopic optical system which projects the image on the inner surface of the visor. The image is reflected to a planar mirror on the helmet and then to the visor again where it is directed into the eye of the viewer and appears to be a part of the exterior scene.
A substantially more complex helmet display system is disclosed in U.S. Pat. No. 4,361,384, to Bosserman. That device combines a thin film flat panel display and a wafer diode image intensifier tube. A plurality of semi-transparent optical elements direct the image from the flat panel display into the eye of the observer. The image intensifier, using some of the same optics, also directs an image into the eye where it can be surperimposed over the images received from the exterior scene. The wearer thus views the exterior scene through both the visor and the flat panel display images. The visor, for reflecting purposes, can be holographic, toric, aspherical or spheric.
A toric combination was disclosed in U.S. Pat. No. 4,026,641, to Bosserman et al. The toric reflector does not appear to be a part of a visor, but rather, appears to be adapted to be mounted on the helmet.
A rather straightforward, helmet mounted system is disclosed in U.S. Pat. No. 4,153,913, to Swift, which utilizes a CRT in combination with a semi-transparent combiner screen. The patent suggests that the combiner plate be worn as goggles or spectacles.
U.S. Pat. No. 4,081,209, to Heller et al., utilizes a more complex optical system in combination with a spherical, semi-reflective combiner surface which may be part of a visor. A prism transmits the image from the source and directs it to the combiner surface which returns the image to the eye of the observer. A refractive interface at an inclined plane with respect to the image source compensates for distortion of the image due to different optical path lengths of different rays.
As will be seen, most prior art helmet visors were surfaces of revolution, usually paraboloids which resulted in a rather bulky visor. Such a design was necessitated by the requirements of tooling for the molding or forming of such visors. A modification to the design of the prior art paraboloidal visor was described in U.S. Pat. No. 3,870,405, to Hedges, which provided for a visor whose inner and outer surfaces were sections of cofocal paraboloids of revolution. Because earlier paraboloidal visors were of substantially constant thickness, images reflected off of the inner and outer surfaces did not have the same focal point and would therefore result in "ghost" images. The Hedges improvement resulted in both images being collimated with no angular difference in the pointing direction, and, as a result, the images appeared to be superimposed on the exterior scene with no loss in sharpness or clarity to the viewer.
A problem with each of the prior art references noted above and presently available helmet mounted display devices has been their size and bulk. Because the helmet member of the helmet mounted display system must be worn by an individual with many complex tasks to perform, usually under great stress and with little spare time in which to accomplish the tasks, the helmet should be as light and compact as possible while still providing superior optical performance. Specifically, bulky and large helmets cause their wearers to tire rapidly, thus shortening the effectiveness of the wearer. Similarly, poor images cause miscalculation as well as hinder the wearer's decision and reaction capabilities.
Consider, for example, the Mostrom reference system disclosed above. The Mostrom reference system includes a pair of visor mounted parabolic mirror sections working with a helmet mounted planar mirror to achieve the proper projection of the optical image into the observer's eye. However, because the parabolic mirror sections are located high on the visor for optical symmetry, the resultant helmet-visor combination tends to be rather large and bulky, requiring a substantial volume to accommodate the optical display system.
To solve the problem of bulk and weight, the present invention discloses that by replacing the single, helmet-mounted planar mirror, which is positioned on the front of the helmet above the wearer's eyes and substantially midway between the parabolic mirrors, with a pair of "fold" mirrors which rotate the images to be displayed to maintain a symmetric, although folded, arrangement of the parabolic mirror surfaces, the volume and size of the helmet and visor can be substantially reduced with improved optical performance and less physical demand on the helmet wearer.